In a prior art, as a pressure sensor for detecting pressure in a medium of relatively high pressure or temperature, as in an internal combustion engine, use is made of a pressure sensor of a liquid confined type wherein a semiconductor pressure detecting element such as a piezo-electric type resistor element, is sealingly stored in a liquid such a silicon oil confined by a diaphragm.
In this kind of the liquid confined type pressure sensor, a change in an environment temperature causes the volume of the confined inside liquid to change. Thus, the pressure of the liquid varies in accordance with the pressure of the liquid. The pressure sensor issues an output signal, which is, also, obtained even when no pressure to be detected exists, due to the fact that a thermal output (outlet value due to the heat) is generated by the existence of the internal pressure. A value of the error has a negative value when the temperature change is low, and has a positive value when the temperature change is high. Furthermore, such an error does not change linearly with respect to a change in the temperature, and is difficult to correct by a usual temperature correction circuit. Thus, it is required that such a non-linearity is reduced as much as possible.
A Japanese Patent Publication No. 4-204130 discloses, for example, a diaphragm defining, at a location adjacent its fixed end, a pair of corrugations constructed as a succession of convex and concave portions, which pair of corrugations function to absorb the change in the inner pressure.
However, such a succession of the convex and concave portions in the prior art device is defective in that they generate the following problems.
As shown in FIG. 9, a thermal expansion of the silicon oil due to its temperature increase causes the inner pressure to be changed, thereby obtaining a deformation in a diaphragm 100. In this case, a curvature of the convex portion 101 is as a whole increased, while a curvature of the concave portion 102 is reduced at its half section located adjacent the convex portion 101, i.e., at its left half section in the drawing. Namely, due to a direct influence of the increase in the curvature of the convex portion 101 to the left half portion of the concave portion 102, this left half section of the concave portion 102 is urged so that its curvature is reduced. Due to such a reduction in the curvature of the left half section of the concave portion 102, a sufficiently large amount of the displacement of the diaphragm can not be obtained. Namely, a change in the inner pressure caused by the temperature change can not be absorbed, resulting in a problem that a desired linearity of the temperature characteristic can not be obtained.
The present invention is conceived in view of the above difficulty, and aims to provide a semiconductor type pressure sensor capable of preventing an output being generated due to a change in temperature, thereby improving the linearity of the output with respect to a change in temperature.